


Back of the Lot

by SprucePines



Category: Gravity Falls
Genre: F/M, Incest, Sibling Incest, Twincest, pinecest - Freeform
Language: English
Status: Completed
Published: 2016-06-08
Updated: 2016-06-08
Packaged: 2018-07-13 20:33:57
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,935
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7136096
Author URL: https://archiveofourown.org/users/SprucePines/pseuds/SprucePines
Summary: <blockquote class="userstuff">
              <p>Mabel tries to get Dipper to do exactly what the back seat is meant for while at a drive-in theater. Please feel free to comment if you like.</p>
            </blockquote>





	Back of the Lot

Dipper looked incredulously up at the giant screen as an equally giant robot vampire strode across it, a painfully obvious miniature town under his feet, badly dubbed voice acting piping through the tinny speaker hooked to his car’s back window. Both he and Mabel had laughed themselves stupid when they saw the title of the b-feature, a Japanese kaiju remake of The Widdlest Wampire. They both wondered how they could make that work. Not that a title like Widowis Wampaiya: Regenesis X Ultra left much to their imagination. Now they were being treated to a vampire mech battling it out with an anthropomorphic wooden stake.

Mabel guffawed next to him. His hand had been holding hers the entire night, through the cat car heist caper The Fast and the Furriest (he didn’t think it was humanly possible to utter one constant “Awwww” through an entire ninety-minute movie, but Mabel somehow managed it), through a trio of classic 1930s slapstick shorts, and now through this unintentionally hilarious (or perhaps it _was_ intentional, for all either of them knew) celluloid “treasure.”

His thumb idly rubbed atop her knuckles, occasionally squeezing. She always responded with a squeeze of her own. Since the two of them had begun their secret relationship together, it had become a bit of silent code for “I love you” when either or both of their parents were around the house. In fact, this time alone was practically their first time alone in several days. Well, not _exactly_ alone, as they were surrounded on all sides by other cars.

A noise to their left grabbed Dipper’s attention. He looked out the window to see the adjacent car rocking up and down, the springs creaking with each motion, its windows fogged up. He felt the blood creep up into his cheeks when he heard a similar noise from the opposite side. This time, both he and Mabel looked to their right. Sure enough, the car on that side was moving up and down, a bit more energetically.

Dipper quickly snapped his head back to look up at the screen, trying his best to shove both the sight and sounds out of this mind. From behind, though, he heard an enthusiastic “WHOO!” Unable to ignore it, the twins looked back to spy a van. But not just any van; this was one they both recognized as being the Love God’s, and it was rocking more ways than one. The rear end was bouncing, one side of the front end was bouncing opposite the rear, plus the entire vehicle was shaking from side to side.

“Well!” he said, clearing his throat as he once again turned back towards the screen. “I guess not everyone comes here for the entertainment.”

Mabel, meanwhile, was still observing the van. “I don’t know,” she said. “Looks like they’re plenty entertained to me.”

Dipper was glad the lights were low. He was pretty sure he was beet red by this point. He coughed again, eliciting some giggles from his sister.

“You know…if you wanted, we could get in on some action ourselves.”

He looked at Mabel aghast. “What?! Are you crazy? Someone might see us.”

“Oh yeah?” she asked. “Then tell me who’s in the car next to us.”

Dipper opened his mouth to speak, but realized that he couldn’t.

“See?” She leaned in close and pressed her lips against his neck. “We don’t get many chances to be alone around the house. Why pass this chance up?” She reached up and brought his face down to look at her, inching their lips closer and closer together.

Just before contact, though, he pulled back, focusing once more on the movie. “We shouldn’t. Really, someone might spot us.”

“Like who, the Love God? Even if he did, we could just say that maybe we accidentally dosed ourselves with that love potion. Besides, I think he’d be far more open-minded about this sort of thing.”

Dipper remained still, ignoring his sister’s joke. Mabel narrowed her eyes at him and pouted a moment before suddenly brightening up. She lifted herself up off the back seat, where she had been sitting on a custom seat cover she had made herself after Waddles had started gnawing on the upholstery. The stitching depicted a cartoon human with an exaggerated posterior aimed at the viewer with a legend underneath. She grabbed the cover and sat back down, draping it over her lap so the legend, “YOUR BUTT GOES HERE,” was laying across her lap.

“Oh, Dipper,” she sang. With a sigh, he turned to look at her. As he looked down at her lap, she wiggled her hips, making it appear that the person on the seat cover was twerking. “You know you want to. Wink wink, nudge nudge, squeaky squeaky.”

Despite himself, he chuckled, but quickly sobered up. “Mabel, that doesn’t even make any sense.”

She stopped her wiggling and glared at him a moment before looking back down at her lap. “I guess you’re right,” she eventually conceded before pulling it off of her and placing it on Dipper’s lap. Before he knew what she was doing, she hopped up and sat down on his covered lap.

“Mabel,” he said, “what are you doing?”

“What’s it look like? Following instructions, you dingus. Oh! Can’t forget to buckle up.” She reached down and pulled the seatbelt across her lap and snapped it into the buckle, pulling it taut and pressing her backside down onto her twin.

Dipper inhaled sharply, taken completely by surprise by their bodies being pushed together. Instantly he felt his member begin to react. Mabel felt it begin to stir, too, and with a devilish grin took hold of both his wrists. “And we could use something to keep my upper half from slamming forward,” she said and placed his hands on her chest.

Dipper felt his pants tighten even more. Mabel looked back at him with a sly smile. “Something tells me your ‘stick shift’ wants to be taken out of neutral.”

He grunted. “Geez, Mabel, that doesn’t make any sense, either. This car is automatic; it doesn’t have a clutch.”

Her eyebrows arched. “Clutch? You mean like you’re doing to my boobs right now?”

Dipper looked at her a moment before he realized that his hands had closed around her breasts on their own and were groping them. He started to loosen his hold, but her hands found his and held them fast. “Admit it,” she said, “you like how they feel.”

“Well…yes…”

“And you would really like to do more, wouldn’t you?”

“Yes, but what if something happened?”

Mabel scoffed. “Dipper, I told you, everyone here is preoccupied—“

Dipper raised his voice. “No, it’s not just that. Even if we had protection, what if it didn’t work and you got pregnant? We wouldn’t be able to keep it a secret from Mom and Dad and they would demand to know who the father is.”

Mabel grimaced and brought one of his hands to her lips, kissing it. “Oh, Dip. I didn’t realize you’d been thinking about that.”

“Yeah, so that’s really why we can’t get frisky or anything like that out here.”

“Ah, believe it or not, that’s where you’re wrong. I just so happen to have thought of this possibility, too, and I have a solution.”

Dipper knit his brow. “You do?” he asked, to which his twin nodded. “What is it?”

With a twinkle in her eye, Mabel unbuckled the seatbelt and half-crawled over the front seat. Dipper had a not unwelcome view of her backside as he heard the click of the glove compartment being opened. While she rummaged inside, she swayed her butt playfully from side to side. A moment later, she pushed herself back holding a pair of objects in her hand. One of them, he recognized as a condom in its wrapper, the other was personal lubricant. Anal lubricant.

Dipper gaped at her. “Are you serious?” he asked.

“You better believe I am, Dip-Dop. I wouldn’t have this if I wasn’t.”

He worked his mouth, trying to find words in light of what his sister was proposing. He settled on a question he knew was bizarre even as he asked it. “Why do you even have that in the glove box?”

“Same reason as the condoms, silly: always be prepared.” She looked at him plaintively. “Please, Dipper. I want to do this. I want to feel you in me somehow.”

Dipper looked from the contents of her hand to his sister’s face and nodded. “Okay,” he said, taking the lube from her. “As long as it’s what you want.” With a giggle Mabel threw herself at him, wrapping her arms around him and kissing him forcefully. As he returned the kiss, Dipper slowly allowed the two of them to fall over on the seat. Mabel’s hand reached between their pelvises and cupped him through his pants, pulling a grunt from deep in his core.

She lifted herself up slightly and swung her body around, placing a knee on either side of her brother’s head. Dipper instantly got a full view up her skirt and he was greeted by a number of cartoon pigs, all posed so that their own hindquarters were aimed at the viewer, their little piggy eyes looking back over their shoulders. He reached up, slipped his fingers into the waistband, and pulled them down her legs.

Meanwhile, Mabel unbuttoned his pants and pulled the zipper down. Using her thumbs, she pushed them down to the middle of his thighs. Even in the low light, she could see a wet spot on his underpants from her earlier shenanigans. She gently kissed him through the cotton of his boxers, causing it to twitch uncontrollably. With a grin, she undid the button fly and snaked a pair of fingers inside, fishing his erection out. She gave the tip a tender smooch before promptly slipping her lips down and around its length, humming appreciatively as she did.

Dipper’s eyes closed as he drew in a staggered breath, lightly tracing his index finger along Mabel’s slit for a moment. He popped the cap off the tube of lubricant and squeezed a large dollop onto his fingertip and reached up between her cheeks, applying it around her rear entrance. As he did so, he pulled her hips further down and began to run his tongue around her lower lips. He danced along her nub a few times, while gently pressing his finger against her ring. Slowly, the muscles began to relax and the tip sank inside.

Mabel made a muffled sound of surprise at the sudden intrusion, even if she was anticipating it. Dipper slowly pushed further in, spreading the lubricant around. Once he managed to fit it all in, he began to slowly retract, pausing just as the cuticle began to reappear before reinserting it. All the while, he continued to pleasure her with his lips and tongue.

Mabel continued to slide her mouth up and down Dipper, using her tongue to gather up the precum he was oozing. He was already stiff when she began, but he seemed to be getting harder still with every passing second. From behind her, she felt her twin completely remove his finger, suddenly making her feel empty. She was about to stop to voice her concern when she felt not one finger, but two, begin to push their way inside her. She relaxed herself and sighed contentedly when they both easily slipped in. Dipper began slow again, allowing his twin to grow accustomed to the invasion. He added more lubricant to his fingers and they were soon sliding in and out easily.

As he was preparing to add a third finger, he felt Mabel pull up off of him. “I’m ready, Dipper,” she said as she got up onto all fours.

“Are you sure?” he asked once more.

“I can’t wait any longer.” She shook her backside at him to illustrate her point. Dipper slid out from underneath her and climbed up onto his knees behind her. He risked a glance around and was relieved to find that the windows had become as fogged as their neighbors. Mabel handed the condom back to him and he tore the wrapper open. Pinching the tip, he carefully rolled the latex down his shaft, then applied another ample supply of lubricant to it, stroking it to slather it up.

Using his other hand to spread her bum a bit wider, he put the tip of his cock right against her hole. Holding it steady, he slowly increased the pressure and watched as it popped into his sister. The ring of muscles constricted around him, gripping him and helping keep him from simply forcing the entire thing inside her. Mabel pressed her face against the seat cushion and groaned.

“You okay, Mabel?” he asked, concerned. He began to draw back, but her hand on his wrist stopped him.

“Y-yeah,” she panted. “Just give me a second. I thought I was ready for you. Guess I was wrong.”

Dipper kept himself still, using a thumb to gently massage the circumference of her hole. Gradually, he felt her relax and she asked for him to start again. He kept his motions extra slow, ready to pause the moment he sensed any discomfort, but after the initial break she didn’t speak up or even tense. Neither of them made so much as a sound until he felt his hips press up against her glutes, when they both released their breaths in one large sigh.

“God, you’re so tight, Mabel.”

“No, it’s you,” she breathed. “You’re so big.”

“I’m not that big,” he protested.

“Oh?” Mabel asked, intrigued. “And who have you been comparing to?”

Dipper instantly blushed. “It’s nothing like that! I just…after gym in the showers…you can’t help but notice, is all.”

“Well, as the one who’s actually _feeling_ it, Boss Mabel hereby declares that you are, indeed, big.”

Dipper laughed at the reminder of how she ran the Mystery Shack for a few days.

“Now, speaking of your prodigious assets?” she continued. “Yeeeah, if you could just go ahead and start giving it to me, that would be greeeaat.”

Dipper blinked. “Really. Mabel?”

Mabel giggled. “Oh, you love it; you know you do.”

“Just don’t tell me to remember the cover sheets on my TPS reports, okay?”

“I thought you got that memo.”

Dipper silenced his sister with a playful, yet firm smack on her bottom, eliciting a yelp. Her body reacted by tensing, gripping him tighter inside as he began to withdraw before pressing back inside. It took a bit of willpower on his part to not simply start thrusting wildly. As amazing as it felt, he knew it wasn’t necessarily as amazing on her end. It wasn’t until he heard her moan softly that he grew a bit bolder and began to pick up his speed and force just slightly.

Mabel was in a bit of sensory overload. She wasn’t used to having anything entering that particular orifice. Even with the amount of lube that Dipper used, she could still feel every inch of his length sliding in and out. She wasn’t yet sure if she really enjoyed it, but it was certainly intense. As her twin sped up a bit, the intensity grew. She felt Dipper lean over her to gently kiss the rim of her ear and whisper her name. She turned her head and hooked her lips to his.

Dipper reached a hand around and found her still wet sex. He traced a circle around it before softly sinking his fingers into her and moving them in time with his thrusting. He rubbed her bump with his thumb, causing her to lurch. She grunted from the combined sensations and felt her orgasm suddenly racing towards her. Her entire body seemed to clench up as she hit the wall. Dipper felt her grip him ever tighter as he thrust a few more times and his own climax rolled through him. As he came down, he fairly collapsed atop his sister and the pair of them fought to catch their breath.

At last, they finally recovered enough to speak. “Holy cow, Mabel,” Dipper said as he pulled back out and nearly fell back on his haunches. “That was incredible.”

Mabel nodded. “I know. I’d never felt anything like that. I wonder what regular sex feels like. Maybe one day soon we’ll be able to find out.”

He smiled, finding himself growing keen on the idea of sex. “Maybe.”

Mabel exhaled. “Hey, Dip, could you lower the window? I think we could use some air.”

Dipper reached for the crank and rolled the window down. Immediately, he was greeted by a pair of excited eyes. He jumped in surprise, screaming. Mabel instantly looked over and froze. Unsure what else to do, Dipper could only continue to roll the window down, exposing the grinning visage of Mayor Tyler Cutebiker. Tyler looked from one to twin to the other, back and forth. Even without saying anything, they could tell by the look in his eyes that he had witnessed enough, if not the entire thing.

After what felt like forever, the mayor broke the stalemate. He slowly extended his arm into the car, his fist closed. The twins both stared at the fist, then back to Tyler’s face. He continued to look at them, grinning. Uncertain, Dipper and Mabel both closed their own hands into fists and brought them up to tough Tyler’s. As their fists met, Tyler finally spoke up.

“Got it.”


End file.
